The present invention relates to a guarding structure for a probe station.
Probe stations are designed to measure the characteristics of electrical devices such as silicon wafers. Probe stations typically include a chuck that supports the electrical device while it is being probed by needles or contacts on a membrane situated above the chuck. In order to provide a controlled environment to probe the electrical device, many of today's probe stations surround the chuck with an environmental enclosure so that temperature, humidity, etc. may be held within predetermined limits during testing. Environmental enclosures protect the device from spurious air currents that would otherwise affect measurements, and also facilitate thermal testing of electrical devices at other-than-ambient environmental conditions. Environmental conditions within the enclosure are principally controlled by a dry air ventilation system as well as a temperature element, usually located below the chuck, that heats or cools the electrical device being tested through thermal conduction.
Many probe stations also incorporate guarding and electromagnetic interference (EMI) shielding structures within or around the environmental enclosures in order to provide an electrically quiet environment, often essential during high frequency testing where electrical noise from external electromagnetic sources can hinder accurate measurement of the electrical device's characteristics. Guarding and EMI shielding structures are well known and discussed extensively in technical literature. See, for example, an article by William Knauer entitled “Fixturing for Low Current/Low Voltage Parametric Testing” appearing in Evaluation Engineering, November, 1990, pages 150-153.
Probe stations incorporating EMI shielding structures will usually at least partially surround the test signal with a guard signal that closely approximates the test signal, thus inhibiting electromagnetic current leakage from the test signal path to its immediately surrounding environment. Similarly, EMI shielding structures may include interconnecting a shield potential to the environmental enclosure surrounding much of the perimeter of the probe station. The environmental enclosure is typically connected to earth ground, instrumentation ground, or some other desired potential.
To provide guarding and shielding for systems of the type just described, existing probe stations may include a multistage chuck upon which the electrical device rests when being tested. The top stage of the chuck, which supports the electrical device, typically comprises a solid, electrically conductive metal plate through which the test signal may be routed. A middle stage and a bottom stage of the chuck similarly comprise solid electrically conductive plates through which a guard signal and a shield signal may be routed, respectively. In this fashion, an electrical device resting on such a multistage chuck may be both guarded and shielded from below.
FIG. 1 shows a generalized schematic of an existing probe station. A probe station 10 includes a chuck 12 that supports an electrical device 14 to be probed by a probe apparatus 16 supported by a platen 18 located above the chuck 12. The chuck is fixedly and/or rigidly interconnected with a tub enclosure 20. The enclosure 20 may be conductive and electrically connected to a guard signal, shield signal, ground signal, or floating. The tub enclosure 20 at least partially surrounds the chuck 12, and hence the electrical device 14.
Multiple electrical devices contained on a silicon wafer may be successively positioned below the probe apparatus 16 for testing by moving the combination of the tub enclosure 20 and chuck 12 laterally. A positioner 22, typically located below the tub enclosure 20, may provide vertical, lateral and/or angular adjustments of the chuck 12. Because the chuck 12 does not move laterally with respect to the tub enclosure 20, the size of the tub enclosure 20 may closely surround the chuck 12, facilitating efficient control of the environment immediately surrounding the chuck 12.
FIG. 2 shows a generalized schematic of another probe station 11. Referring to FIG. 2, where numerals common with FIG. 1 represent similar elements that perform similar functions, the probe station 11 includes the chuck 12 that supports the electrical device 14 to be probed by the probe apparatus 16 that extends through an opening in the platen 18. Rather than enclosing the chuck 12 in the tub enclosure 20, an outer shield box 24 provides sufficient space for the chuck 12 to be moved laterally by the positioner 22. Because the chuck 12 may freely move within the outer shield box 24, a suspended member 26 electrically interconnected to a guard potential may be readily positioned above the chuck 12. The suspended guard member 26 defines an opening that is aligned with the opening defined by the platen 18 so that the probe apparatus 16 may extend through the guard member 26 to probe the electrical device 14. When connected to a guard signal substantially identical to the test signal provided to the probe apparatus 16, the suspended guard member 26 provides additional guarding for low noise tests. Such a design is exemplified by EP 0 505 981 B1, incorporated by reference herein.
To provide a substantially closed environment, the outer shield box 24 includes a sliding plate assembly 28 that defines a portion of the lower perimeter of the shield box 24. The sliding plate assembly 28 comprises a number of overlapping plate members. Each plate member defines a central opening 30 through which the positioner 22 may extend. Each successively higher plate member is smaller in size and also defines a smaller opening 30 through which the positioner 22 extends. As shown in FIG. 2, the sliding plate assembly 28 is included to permit lateral movement of the positioner 22, and hence the chuck 12, while maintaining a substantially closed lower perimeter for the shield box 24.
One drawback of the probe station 11 depicted in FIG. 2 is that the large space enclosed by the outer shield box 24 results in some difficulty in efficient maintenance of a controlled environment within the outer shield box 24. Not only does more heat have to provided to or removed from the outer shield box 24 when testing an electrical device across a range of temperatures, but dry air ventilation must be provided to the shield box 24 in large quantities. The air currents generated by such a ventilation system may sometimes interfere with accurate measurements taken by the probe station 11.
What is desired, then, is a probe station that provides a quiet electrical environment surrounding the chuck and may readily maintain a controlled environment.